Placental Site Trophoblastic Tumour (PSTT)

Epidemiology and Etiology:

· Rare (~200 cases)

· Intermediate trophoblast differentiation

· Normally found at implantation site (residual placental site / implantation site nodule) and placental membranes

· Reproductive age women

· Preceded by:

· Normal pregnancy (50%)

· May be up to 18 years after pregnancy

· Spontaneous abortion (16%)

· Hydatiform mole (5-10%)

Common sites:

· Myometrium

· Endomyometrium

· Cervix (occasionally)

Gross features:

· Uterine enlargement

· Several mm to large bulky mass up to 10cm

Histologic features:

· Infiltrating sheets and cords

· Separate and split apart individual smooth muscle fibres

· Confluent masses of trophoblastic cells

· (exaggerated placental site is microscopic in size, composed of intermediate trophoblastic cells separated by masses of hyaline, and usually is admixed with decidua and chorionic villi. Also it contains larger numbers of multinucleated trophoblastic cells than PSTT)

· polygonal cells

· moderate amount of dense amphophilic, eosinophilic or clear cytoplasm

· may be spindled

· nuclear pleomorphism may be marked or monomorphic

· scattered multinucleated cells

· extracellular fibrinoid material (characteristic)

· mitotic rate variable

· mitotic figures usually not seen in exaggerated placental site

· less than 1 / 10 HPF to > 30 / 10 HPF

· usually 0-6 / 10 HPF

· atypical mitoses (90%)

· coagulative necrosis often

· hemorrhage often

· inflammation often

· Distinction from normal exaggerated placental implantation site trophoblast may be difficult

· Mel-Cam and Ki-67 are helpful to detect increased proliferation

· Depply invasive (60%)

· May extend to serosa and perforate

Immunophenotype:

Marker:	Sensitivity:	Specificity:
Human placental lactogen (strong, diffuse)		Positive in PSN (focal)
Beta-HCG (focal, weak)
Mel-Cam (CD 146) (strong, diffuse)		Positive in PSN (focal)
Ki-67 (10-15%)		Near zero in normal and exaggerated placental site > 40% in chorioCA
PLAP (focal, weak)		Positive in PSN (diffuse)
Alpha-inhibin (strong, diffuse)		Positive in PSN
Cytokeratins (diffuse)

Molecular features:

·

Other features:

· Often misdiagnosed as pregnancy

· Beta-HCG may be high (80%)

· Most benign

· Good prognosis if:

· Localized (stage I or II)

· Less than 2y interval from prior pregnancy to diagnosis

· Poor prognosis if:

· More than 4y interval from prior pregnancy to diagnosis

· Advanced stage or lung involvement

· Overall 10% mortality

· Metastases in 10-15%

References:

· Kumar V, Fausto N, Abbas A. Robbins & Cotran Pathologic Basis of Disease, Seventh Edition. 7th ed. Saunders; 2004.